Cancer is characterized by cells that are aggressive, invasive, and often metastatic. Only a subset of tumor cells possesses the ability to metastasize, and yet it is the metastatic sub-population that makes cancer a lethal disease. Therefore, it is critical to understand how this population of cells differs from normal tumor cells. If we can find markers that are specific to the metastatic cells, then there is a hope that we can kill them, or at least restrict them. This proposal will focus on the precise molecular function of palladin and a-actinin, two proteins that have both been implicated as playing critical roles in cell motility and particularly in cancer metastasis. The novel protein palladin is a multi-domain, actin-associated protein that is highly conserved among vertebrate species. The palladin knockout mouse displays a phenotype of embryonic lethality, confirming that palladin plays an essential role in mammalian embryogenesis, and palladin null cells have defects in both motility and adhesion. a-Actinin is also involved in actin-crosslinking and serves as a platform for the assembly of multi-component complexes involved in cell signaling and actin reorganization. Recently, a mutation in human palladin (the P239S mutation) was implicated in an inherited form of pancreatic cancer, suggesting that impaired palladin function may contribute to a highly invasive human disease. This mutation also occurs in a region of palladin required for binding to a-actinin. Therefore palladin and a-actinin are both functionally and physically linked to both normal and pathological cell motility. Current research is focused on using palladin or a-actinin levels as prognostic biomarkers. Yet, our hypothesis is that the palladin/a-actinin complex is the relevant biomarker for metastatic cancer and hope that this work will lead to improved prognostic markers for the deadliest of cancers. Currently the precise molecular role of this complex in organizing the actin cytoskeleton is unknown. We propose to solve the structure of the complex in solution using nuclear magnetic resonance, which will in turn guide our rational mutagenesis of palladin to disrupt the complex. Then we will use transfection of cultured breast cancer cells to characterize the biological activity of palladin mutants that are deficient in their ability to bind a-actinin. Together, these aims will allow us to determine the biological significance of the interaction between palladin and a-actinin in cell motility and invasion.